Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier recording materials include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc.
For aqueous dye-based inks, the dyes needs to be sufficiently soluble in water to prepare a solution that is capable of producing adequate density on the receiving element and stable for extended periods of storage without precipitation.
To generate full color prints via ink jet printing, ink sets comprising at least cyan, magenta and yellow inks are normally utilized. In addition a black ink is often added to enhance the printing of text and darker colors. The range of colors that can be produced with a given set of inks defines the color gamut of that ink set. For the production of high quality photo-realistic images via ink jet printing, ink sets with a large color gamut are preferred. In addition, it is important that the ink sets produce images with good fastness, especially to light.
The choice of the colorants in ink jet systems is critical for both light fastness and color gamut. The color gamut of an ink set is controlled primarily by the spectral absorption characteristics of the component dyes. The primary dyes (e.g., cyan, magenta and yellow) should only absorb light of the required wavelengths (i.e., have relatively narrow absorption bands) and not overlap excessively with the dyes in the complementary inks. In addition to the light absorption characteristics of the dyes, the maximum print density achieved from the ink is another important factor to provide a high color gamut. The higher the print density the greater the color gamut.
For printing black and white images with high detail, a black ink can be used to produce different levels of gray. In this case, is important to maintain a near neutral tone over the entire density scale. One method of assessing the tone of a black or gray patch is to measure the printed image in CIELAB color space, specifically a* and b* values.
It is also important to use inks in the ink jet ink set which are resistant to light fade to preserve the printed image. Of particular importance is the light fastness of the inks when printed on recording materials which is subsequently sealed with a laminating film. Lamination is done to protect the printed image from physical damage and also to further enhance the stability of the printed image against light fade and fade due to environmental pollutants such as ozone. Laminate films are available in a variety of forms and can be categorized based on their mode of adhesion to the printed recording materials. These forms include heat activated laminates, thermal laminates and pressure sensitive laminates. Laminate films can contain ultraviolet light absorbing compounds which can in part protect the printed dyes from fade due to ultraviolet light.
U.S. Pat. No. 5,439,517 describes the addition of yellow dyes (such as C.I. Direct Yellow 132 and C.I. Direct Yellow 86) to black ink containing a black disazo dye to adjust the tone of the black ink on the receiving element. However, the light fastness of this class of black dyes on a laminated receiving element is not as high as one would like. U.S. Patent Application 2002/0121219 describes a mixture of two black dyes; Pacified Reactive Black 31 and C. I. Direct Black 168 to improve the black ink tone and thus provide a more neutral image. However, the light fastness of Direct C. I. Black 168 on a laminated receiving element is not as high as one would like (as shown in the comparative examples below). Japanese Patent Publication No. 59-129272 describes black inks for ink jet printing which contain both black dyes and black pigments. U.S. Pat. No. 6,342,096 describes combinations of black pigment, a black dye, and cyan dye to provide a more neutral tone. However the light fastness of the example black dyes described (for example C. I. Food Black 2, C. I. Direct Black 195, C.I. Direct Black 168) are not a high as one would like. In addition, ink sets combining black pigment inks with cyan, magenta and yellow dye inks can produce images with undesirable image artifacts. An example of such an artifact is a gloss difference in areas printed with black pigment inks on a glossy receiving element versus areas printed only with dye inks.
Metal complex black dyes have been disclosed with good light fastness on receiving element that are subsequently laminated after printing. For example, U.S. Pat. No. 5,725,641 discloses Pacified Reactive Black 31 which is a copper complex black dye. U.S. Pat. No. 6,302,949 discloses metal complex bisazo black dyes. U.S. Patent Application No. 2001/0027734A1 discloses metal complexes of trisazo black dyes. However, used as the sole colorant in an ink jet ink, these metal complex black dyes do not provide as neutral a tone as a function of print density as one would like.
It is an object of this invention to provide a black ink for ink jet printing capable of producing images with near neutral tone over a range of densities while providing high light fastness when printed on a variety of recording elements that are subsequently laminated. Another object of this invention is to provide a black ink of a color ink jet ink set with near neutral tone over a range of densities while providing high light fastness when printed on a variety of recording elements that are subsequently laminated.